There exists a demand in this specific field to improve the quality of the communications over the telephone lines and, at the same time, to expand the range of services offered to the users. In this respect, it can be mentioned that token-operated apparatuses installed in public telephone boxes produce noise (spurious signals) in the audio band each time that a token is accepted. This is due to a reversal of the polarity brought about at the line ends. Furthermore, in recent years, complex telephone services, such as "videotel", are gaining widespread acceptance. Such services involve transmission of trains of digital information over the telephone lines.
To fill such increasingly more sophisticated demands, a constant effort to improve the quality of transmission over telephone lines has become mandatory. This effort concentrates, on the one side, on improving the structural quality of each line or of the telephone network as a whole. In fact, many national telephone companies are known to be involved in restoring their telephone networks through the use of fiber optics. On the other side, the improvement effort is directed toward the suppression of any noise in the audio band as well as interference in the band employed for transmitting digital information. To that aim, attention has been focused on the circuit devices which operate to interface the telephone subscriber lines. For example, it is a well-recognized fact that a controlled or "soft" reversal in the supply polarity to the telephone subscriber line could ensure suppression of the aforesaid noise caused by public telephones.
As used herein, "polarity reversal" or "battery reversal" means to exchange the polarities of the TIP and RING wires in a two-wire telephone line. In other words, to reverse polarity is to cause the TIP wire to take the voltage of the RING wire, and vice versa. It would indeed be appreciated if the interfacing telephone circuit connected to the line could perform a soft reversal of the supply polarity to the line.
However, interface telephone circuits have been implemented heretofore by integrated circuits which tend to be complicated both in construction and operation, and to which new functions cannot be integrated in a simple manner without compromising in some other areas of the circuit. In the specific instance considered, the reversal of polarity to the line, i.e. exchanging the TIP and RING wires, may cause a malfunction instead of fostering the suppression of line noise. Thus, no monolithically integrated telephone circuits are known at present which can reverse the supply polarity to the subscriber line in a soft manner.
An underlying technical problem of this invention is to provide an interfacing telephone circuit which is monolithically integrated and adapted to drive a telephone subscriber line, having further such structural and functional features as to allow a soft reversal of the line polarity, thereby overcoming the limitations of prior art approaches.